TalcTalc is the softest mineral, demonstrated by its position at the bottom of Mohs' Scale of Hardness with a relative hardness value of 1. It has a soapy, greasy feel.
Talc is formed by the hydrothermal alteration of ultrabasic rocks, or low grade thermal metamorphism of siliceous dolomites.
Most people know talc as the primary ingredient in talcum powder. However, talc is an important industrial mineral. Its resistance to heat, electricity and acids make it useful for lab counter tops and electrical switchboards. It is an important filler in paints, rubber and insecticides.
Talc often replaces other minerals atom by atom to form pseudomorphs, taking the form of the replaced mineral. Thus, a specimen of what appears to be milky quartz would actually be talc, having a soapy feel and being able to be scratched with a fingernail.
Chemical composition - Mg